In topology, a branch of mathematics, the loop space ΩX of a pointed topological space X is the space of (based) loops in X, i.e. continuous pointed maps from the pointed circle S1 to X, equipped with the compact-open topology.
The set of path components of ΩX, i.e. the set of based-homotopy equivalence classes of based loops in X, is a group, the fundamental group π1(X).
The iterated loop spaces of X are formed by applying Ω a number of times.
There is an analogous construction for topological spaces without basepoint.
The free loop space of X is often denoted by
As a functor, the free loop space construction is right adjoint to cartesian product with the circle, while the loop space construction is right adjoint to the reduced suspension.
This adjunction accounts for much of the importance of loop spaces in stable homotopy theory.
(A related phenomenon in computer science is currying, where the cartesian product is adjoint to the hom functor.)
The loop space is dual to the suspension of the same space; this duality is sometimes called Eckmann–Hilton duality.
is the set of homotopy classes of maps
This homeomorphism is essentially that of currying, modulo the quotients needed to convert the products to reduced products.
does not have a group structure for arbitrary spaces
are pointed, and the aforementioned isomorphism is of those groups.
sphere) gives the relationship This follows since the homotopy group is defined as
and the spheres can be obtained via suspensions of each-other, i.e.